1. Field of the Invention 
The present invention relates generally to a nitride semiconductor device comprising a gallium nitride semiconductor layer and its fabrication process, and more particularly to a nitride semiconductor device comprising a gallium nitride semiconductor layer formed on a heterogeneous substrate, wherein light is emitted in a plurality of colors having different light emission wavelengths from an active layer.
2. Related Art
In recent years, nitride semiconductor devices comprising gallium nitride semiconductor layers have been put to diverse uses and available in every aspect of daily life as light-emitting diodes (LEDs) emitting light in various colors and wavelengths such as blue light, green light, white light and ultraviolet light. In particular, white LEDs have high promise for the coming generation of lighting systems superseding fluorescent lamps.
In widespread nitride semiconductor devices comprising gallium nitride semiconductor layers, the first conduction (n) type gallium nitride semiconductor, an active layer and the second conduction (p) type gallium nitride semiconductor layer are stacked on the C plane of a sapphire substrate. The reason such nitride semiconductor devices have wide applications is that gallium nitride is grown on the sapphire C plane with relative ease, so that the crysallinity of gallium nitride is improved. To put nitride semiconductor devices comprising such gallium nitride semiconductor layers to practical use, it is of vital importance that the crystallinity of gallium nitride is satisfactory. The then used gallium nitride has a wurtzite crystal structure, and the growth plane for gallium nitride is the C plane.
As the nitride semiconductor light-emitting devices have wide applications as described above, the attention of users is directed to improvements in the light emission outputs of nitride semiconductor devices and light emissions of different light emission wavelengths from a single nitride semiconductor device. In efforts to address such requests, it has been proposed to fabricate unique light sources by adding some contrivances to the construction of an active layer.
JP-A 9-331116 teaches that asperities are formed on the surface of an indium (In)-containing quantum well active layer so that carriers are confined in the interface between the active layer and a clad layer in both the longitudinal and lateral directions, producing a quantum box effect. The publication then says that a plurality of light emission peaks appear at an interval of 1 meV to 100 meV in emission spectra from the light-emitting device, contributing to improvements in light emission outputs. JP-A 2001-28458 teaches that a multiple quantum well structure having a first well layer and a second well layer is formed in a longitudinal direction to substrate, so that light emissions having different light emission wavelengths or colors are obtained from the respective well layers. In this case, blue light is emitted from the first well layer near an n-type clad layer, and yellow light is emitted from the second well layer near a p-type clad layer.
The nitride semiconductor light-emitting device set forth in JP-A 9-331116 allows a plurality of light emission peaks to appear through the asperities formed on the active layer. However, this arrangement is basically designed to improve the light emission output of the same color; in other words, it is not designed to give out light emissions of different colors. A problem with this light-emitting device is that when used alone, it is not possible to give out light emissions having different light emission wavelengths or colors. On the other hand, the nitride semiconductor light-emitting device set forth in JP-A 2001-28458 is designed to obtain light emissions having different light emission wavelengths or colors. For the multiple quantum well structure with the first and second well layers stacked together, however, the well layers for giving out light emissions of different light emission wavelengths should be located with a barrier layer interposed between them, offering a problem that there is an increased space in the stacking direction.
In view of such problems, one object of the invention is to provide a nitride semiconductor device comprising gallium nitride semiconductor layers formed on a heterogeneous substrate, wherein light emissions having different light emission wavelengths or colors are given out of an active layer and mixed together to obtain light emissions of various colors.